:construction_worker: removing Clang 3.9 and 4.0
:construction_worker: removing Clang 3.9 and 4.0

These versions are still not working properly: https://travis-ci.org/nlohmann/json/jobs/251675281

nlohmann/json,hunter-packages/json,theodelrieu/json,nlohmann/json,hunter-packages/json,kaidokert/json,hunter-packages/json,kaidokert/json,nlohmann/json,theodelrieu/json,kaidokert/json